The present invention relates to tube bending and, more particularly, to variable geometry tooling employing segmented dies to effect this bending.
Numerous bending methods have been developed over the years, but generally speaking, most are variations of a few basic processes. The die-press method, wherein the tube is laid across a plurality of supporting dies and then subjected to the pressure exerted by a movable forming die, is useful to form bend angles up to about 120.degree.. The foil method of bending usually employs three triangularly arranged rolls, the center one of which is adjustable. The workpiece is fed between the outer fixed driven rolls and the adjustable roll to form bends up to 360.degree.. The compression method utilizes a stationary bending die and a movable pressure die. The pressure die traverses the periphery of the bending die, wiping the workpiece into a groove of the bending die to form bends with angles up to 180.degree.. Rotary draw bending is similar to compression bending except the bending die rotates and the pressure die is either stationary or movable.
Currently, rotary draw bending machines form the bends in much of the 1 to 3 inch tubing used in high pressure steam boilers for the electric power generating industry. These machines use a range of dies depending on the outside diameter of the tube and the radius of the bend required. These machines make one bend at a time and the workpiece must be sequentially re-positioned accurately for multiple bends which are made for example in a S-shape. Frequently, such bends must be individually inspected and slightly adjusted to the correct geometry in an additional "check-and-set" operation. As a result, this method is both time and labor consuming.
In comparison, for multiple bends, die-press forming is three to four times faster than rotary draw bending because all forming is done in one die-press stroke. Because of the fixed nature and precision of the supporting dies, no subsequent check-and-set is normally required. However, a different set of dies is required for each tube diameter and bend configuration. This results in a large inventory of dies typically costing from $3,000 to $7,000 and taking from four to six weeks to obtain. The less efficient rotary draw benders are still used because of the many different bends required in different boiler designs. It is cost prohibitive to have a die available for every possible bend configuration.
Thus there is a need in the power generating industry for a cost effective tube bending apparatus and method which is accurate and time efficient.